Tuned h.-f. transformer with improved balanced output



A ril 24, 1962 w. s. KRAKAU ETAL 3 TUNED H.F. TRANSFORMER WITH IMPROVED BALANCED OUTPUT Filed June 25, 1959 INDUCTIVE COUPLED CAPACITIVE COUPLED OUTPUT VOLTAGE VOLTAGE VOLTAGES F|G.3 I I7 4 l6 1 30- f g C I '34 I I 2| 33 ;,2| I I I I a I I 20'' I ,1 I 22" I: I

I T j zl I I 1 i I 1 I8 I: I

I l I I I I 32 I; 37 INVENTOR, 9 w. s. KRAKAU 4 y T. E. OLVER ATTORNEY.

United States Patent The present invention relates to a tuned high-frequency transformer with balanced output, and more particularly to a transformer of this type utilizing a novel arrangement of electrostatic shields to reduce undesired capacitive coupling.

In utilizing such a high-frequency transformer, it is frequently desired that the coupling between the input and output windings be essentially inductive. To accomplish this, it becomes necessary to minimize the capacitive coupling between these windings. capacitive coupling is of particular importance in the design of a transformer to operate as an input to a symmetrical or balanced circuit such as a balanced mixer, discriminator, or push-pull amplifier.

Accordingly, it is an object of the present invention to provide a tuned high-frequency transformer having a minimum of capacitive coupling between the input and output windings.

It is a further object of the present invention to provide a high-frequency transformer capable of a high degree of balance in the induced voltages of the separate portions of the secondary winding.

It is still another object of the present invention to provide a compact high-frequency transformer that is easy to manufacture and small in size.

The transformer of this invention accomplishes this and other objects by a novel arrangement of electrostatic shields. More specifically, a first electrostatic shield is placed so that it is'surrounded by the primary winding, and a second electrostatic shield, shaped to fit the transformer can, is placed between the primary and secondary windings, perpendicular to the transformer axis. The secondary winding comprises two halves wound symmetrically to the second electrostatic shield.

The exact nature of this invention as well as its advantages will be readily apparent from consideration of the following specification taken in connection with the annexed drawings in which:

' FIGURE 1 is a schematic circuit showing of inductive and capacitive coupling in a double-tuned high-frequency transformer connected to a balanced network;

FIGURES 2A, 2B and 2C are vector diagrams of voltages developed in the circuit of FIGURE 1;

FIGURE 3 is an elevational view, partially in section, of the present invention;

FIGURE 4 is a section of the invention taken on line 44 of FIGURE 3 looking in the direction of the arrows; and

FIGURE 5 is a detail showing of the center electrostatic shielding plate forming a part of the structure of FIG- URES 3 and 4.

Referring now to FIGURE 1, there is shown a circuit in which a transformer 11 serves as an input to a balanced network 12. In actual practice it is desirable that the coupling between input winding :13 and output winding 14 of transformer 11 be essentially inductive so that the voltages V and V will be equal in magnitude and opposite in phase. This is generally not achieved because of the capacitive coupling, C and C", betweenthe two windings.

The vector diagrams of FIGURES 2A, 2B, and 2C are helpful in illustrating the unbalance in the circuit of FIG- URE 1 caused by the coupling capacitance. FIGURE The reduction of "ice 2A shows the vector relationship between the output volttages V and V and the input voltage V when purely inductive coupling exists between the input (primary) and output (secondary) windings. This relationship, in which the output voltage is in phase quadrature with the input voltage, follows a well known principle and is due to the fact that the field producing inductive current is in phase quadrature with the current producing input voltage. FIGURE 23 shows the relationship between output voltage V and the input voltage V when capacitive coupling exists in addition to the inductive coupling. The superposition of the vector diagrams shown in FIGURES 2A and 23 results in the vector diagram of FIGURE 20 in which V and V are the resultant vector output voltages. This latter figure illustrates how the capacitive coupling causes unbalance in an inductively coupled transformer. It should be understood that this presentation is simplified since in an actual transformer the coupling capacities are distributed, making an exact calculation almost impossible.

The foregoing discussion shows how important it is that the capacitive coupling, as far as possible, be eliminated when a transformer is to be used as an input to a balanced circuit.

The transformer structure in accordance with this invention serves as a tuned high-frequency input to a balanced mixer having a center frequency of 10 mo. and a bandwidth of 1 mo. Its preferred embodiment, as shown in FIGURES 3 and 4, comprises a transformer shield can 16 having an opening 17 in its upper end. Its bottom end is closed by a bottom insulating board 18. A top insulating board 19 lies in the upper portion of shield can 16 substantially parallel to bottom insulating board 18.

The shield can 16 contains a transformer comprising primary winding 20 and secondary winding 22 wound upon a hollow cylindrical coil form 30 having top and bottom ends atfixed to top insulating board 19 and bottom insulating board 18, respectively. Powdered iron cores 28 and 29 are located within the hollow portion of coil form 30 and are surrounded by primary winding 20 and secondary winding 22, respectively. Apertures 31 and 32 are provided in the top and bottom insulating boards, respectively, to permit access to these cores.

The ends of the primary and secondary windings of the transformer are connected to respective conductive sup 7 porting rods 21. As shown, there are four of these rods, each having one end 37 passing through an opening 9 in the bottom insulating board 18, to form terminals for mounting the transformer structure upon a chassis or other support and as electrical connections to external circuit components. The other end of each supporting rod 21 is anchored in top insulating board 19.

To prevent electrostatic coupling between primary winding 213 and secondary winding 22, without interfering with the magnetic coupling, electrostatic shielding members are provided. More specifically, a center electrostatic shielding plate 23 is interposed between the primary winding and secondary windings perpendicularly to the longitudinal axes of those windings to eliminate the electrostatic coupling between them; and an electrostatic shield collar 33 is placed underneath the primary winding, between it and coil form 30, to avoid the electrostatic coupling through the cores inside of the coil form.

As shown in FIGURE 5, shielding plate 23 has a large opening 24 through which coil form 30 passes, four small holes 25 through which rods 21 pass, a slot 35 for preventing the shielding plate from acting as a shorted turn, and a ground terminal 26 serving as a common ground for the transformer structure. Insulating sleeves 27 fit in holes 25 to prevent electrical contact between shielding plate 23 and rods 21.

Electrostatic shield collar 33 has a longitudinal slot 34 for preventing it from acting as a shorted turn, and a tab 36 for Connecting it to .common ground terminal 26.

An external ground lead 36 passes through bottom insulating board 18 andiis connected to common ground terminal 26. The center-tap dil of secondary winding 22 is also connected to this common ground terminal.

To further improve the balance characteristics of the transformer, the secondary winding is formed of two halves wound symmetrically to the center shielding plate 23, one of said two halves being Wound upon the other, so that the two halves are equally spaced from the center shield, thus equalizing the stray distributed capacities to ground of each half.

In summary, the presently disclosed high-frequency transformer, by achievinga high degree of balance in the induced voltages in the two halves of its secondary winding, provides a definite advancement in the transformer art. This advancement is primarily attained by the novel arrangement of electrostatic shielding members and by the symmetrical placement of the halves of the secondary plate. More specifically, the placement of the halves of the secondary winding permits the stray distributed capacities to ground of each half to be equalized, and the arrangement of the shielding members prevents electrostatic coupling between the primary and secondary winding, both through space and through the cores within the coil form.

The foregoing disclosure relates to a preferred embodimerit of the invention, Numerous modifications or alterations may be made therein without departing from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

1. In atuned high-frequency transformer with balanced output: an outer shielding housing, a coil form supported within said housing, a primary winding wound upon a portion of said coil form, a secondary winding formed of two substantially identical halves wound upon a second portion of said coil form, one of said two halves being wound upon the other, said secondary winding being axially aligned with said primary winding so that the two halves of said secondary winding are substantially symmetrical with respect to said primary winding, an electrostatic shielding plateinterposed between said primary and secondary windings, said two halves of said secondary winding being wound symmetrically to said shielding plate and being equally spaced therefrom, and a shielding collar positioned underneath said primary winding between it and said coil form, 'a common ground terminal, said shielding collar and said shielding plate connected to said ground terminal.

2. The high frequency transformer of claim 1 further comprising first and second powdered-iron cores positioned within said coil form and surrounded by said primary and secondary windings, respectively.

3. The high frequency transformer of claim 2 further comprising a plurality of conductive rods, each of said rods having an end passing through the bottom portion of said housing, the other end of each rod being supported in said housing, said rods being connected individually to terminals of said primary and secondary windings.

4. In a tuned high-frequency transformer with balanced output: a transformer shield can; a bottom insulating board closing ofi the bottom portion of said can; a top insulating board positioned within the upper portion of said can substantially parallel to said bottom board; a hollow cylindrical coil form having top and bottom ends afiixcd to said top and bottom boards, respectively; a primary winding wound upon the upper portion of said winding with respect to the center electrostatic shielding d coil form; a secondary winding formed of two halves wound upon the lower portion of said coil form, one of said two halves being wound upon the other in a bifilar manner; first and second powdered iron cores positioned within the hollow portion of said coil form and surrounded by said primary and secondary windings, respectively;

supporting conductive rods, each of said rods having an end passing through said bottom board, the other end of each rod being anchored in said top board, said primary and secondary windings having ends connected to respective rods; an electrostatic shielding plate interposed beween said primary and secondary windings and having a common ground terminal, said two halves of said secondary winding being wound symmetrically tosaid shielding plate and being equally spaced therefrom; and a shielding collar coaxial with said primary winding and positioned between it and said coil form, said shielding collar having a tab for connection to said common ground terminal, said secondary winding having a center-tap connectedto said common ground terminal.

5. A tuned high-frequency transformer with balanced output comprising a transformer shield can having an opening in its upper end; a bottom insulating board closing oil the bottom portion of said can and having openings therein; a top insulating board having similar openings and positioned within the upper portion of said can substantially parallel to said bottom board; a hollow cylindrical coil form having top and bottom ends affixed to said top and bottom boards, respectively; a primary winding wound upon the upper portion of said coil form; a secondary Winding wound upon the lower portionof said coil form; first and second powdered iron cores positioned within the hollow portion of said coil form and surrounded by said primary and secondary windings, respectively; said top and bottom boards having apertures to permit access to said cores; four supporting conductive rods, each of said rods having an end passing through one of said openings in said bottom board, the other end of each rod being anchored in said top board, said primary and secondary windings having ends connected to respective rods; a center electrostatic shielding plate interposed between said primary and secondary windings perpendicularly to the longitudinal axes of these windings, said shielding plate having a large opening through which said coil form' passes, four small holes through which said rods pass, a radial slot for preventing said plate from acting as a shorted turn, and a ground terminal serving as a common ground; insulating sleeves fitting in said holes in said plate to prevent electrical contact between said shielding plate and said rods; an electrostatic shielding collar positioned underneath said primary winding between it and said coil form, said shielding collar havingalongitudinal slot for preventing it from acting as a shorted turn and a tab for connecting it to said common ground terminal; an external ground lead passing through said bottom board and connected to said common ground terminal, said secondary winding having a center-tap connected to said common ground terminal, said secondary windingbeing formed of two halves wound'symmetrically to said shielding plate, one of said two halves being wound upon the other, said two halves being equally spaced from said shielding plate.

References Cited in the file of this patent UNITED STATES PATENTS 1,839,038 Kronmiller Dec. 29, 1931 2,219,396 Plebanski Oct. 29,1940 2,494,579 Pimlott Jan. 17, 1950 2,535,203 Guthman Dec. 26, 1950 2,878,441 Rogers Mar, 17, 1959 

